Arrays of electrodes for neural stimulation are commonly used for a variety of purposes. Some examples include: U.S. Pat. No. 3,699,970 to Brindley and “The Sensations Produced by Electrical Stimulation of the Visual Cortex” by G. Brindley and W. Lewin, J. Physiol (London) 196:479-493:1968. Brindley's paper and patent describe an array of cortical electrodes for visual stimulation. One cortical electrode is used for each light percept. Each electrode is attached to a separate inductive coil for signal and power. U.S. Pat. No. 4,573,481 to Bullara describes a helical electrode to be wrapped around an individual nerve fiber. U.S. Pat. No. 4,628,933 to Michelson describes an electrode array for retinal stimulation. U.S. Pat. No. 4,837,049 to Byers describes spike electrodes for neural stimulation. Each spike electrode pierces neural tissue for better electrical contact. U.S. Pat. No. 5,215,088 to Norman describes an array of spike electrodes for cortical stimulation. Each spike pierces cortical tissue for better electrical contact. U.S. Pat. No. 5,109,844 to de Juan describes a flat electrode array placed against the retina for visual stimulation. U.S. Pat. No. 5,935,155 to Humayun describes a retinal prosthesis for use with the flat retinal array described in de Juan.
In addition to the electrode arrays described above, there are several methods of mapping a high resolution camera image to a lower resolution electrode array. U.S. Pat. No. 6,400,989 to Eckmiller describes spatio-temporal filters for controlling patterns of stimulation in an array of electrodes. The assignee of the present applications has three related U.S. patent application Ser. No. 09/515,373, filed Feb. 29, 2000, entitled Retinal Color Prosthesis for Color Sight Restoration; Ser. No. 09/851,268, filed May 7, 2001, entitled Method, Apparatus and System for Improved Electronic Acuity and Perceived Resolution Using Eye Jitter Like Motion; filed on current date herewith, entitled User Directed Pixel Re-Mapping. All three applications are incorporated herein by reference.
The density of neural tissue is far greater than currently available electrodes arrays. Using artificial vision as an example, the human retina has approximately 4,000,000 receptors. Further those receptors are not evenly distributed, but are far denser near the fovea, at the center of the retina. The spacing of receptors near the fovea is approximately 5 μm. The best know technology for producing electrodes capable of stimulating retinal neurons requires 40 μm electrodes with 20 μm spaces. Other neural tissues, such as cortical tissue, is about the same scale as retinal tissue. Other neural tissue, such as that found in the optic nerve is far denser. Obtaining a high resolution image is simple and inexpensive using charge coupled device (CCD) cameras. While the mapping systems described above help present the most relevant data given the limited resolution of the electrode array, they do not increase the perceived resolution for the user of a visual prosthesis. A method is needed to direct a high resolution image to a lower resolution electrode array while achieving the highest possible perceived resolution to the individual stimulated by the electrode array.
It is further advantageous to reduce the power needed to stimulate a neuron. As stated above, the smallest possible electrode spacing with current technology is 40 μm electrodes with 20 μm spaces. This assumes a minimal charge requirement. The charge needed varies from retinal to retina, varies across the surface of the retina, and varies over time as retinal disease progresses. Charge density is the charge transferred by an electrode, divided by the surface area of the electrode. As charge density increases, electrochemical reactions at the electrode surface become more intense leading to dissolution of the electrode. It is therefore, advantageous to stimulate neurons with the minimum charge necessary.